An autonomous moored profiler (“AMP”) is a tethered platform typically used for carrying scientific measuring instruments through bodies of water for measuring characteristics of the water. Such characteristics include those of both biological and non-biological material in the water. The measuring instruments may be any combination of mechanical, electrical, optical, chemical, and biological sensors suited to the purpose. The AMP moves the measuring instruments through the water vertically, so that the measuring instruments are able to sample the water at various depths, providing a “profile” of the water.
Motion of the AMP through the water is controlled by a flexible cable. Since the cable is flexible, it can sustain only tensile forces, i.e., it can only exert pulling forces. To allow the AMP to cycle both upwardly and downwardly in the water, the AMP is made buoyant, so that there is an upwardly directed force on the AMP which acts in opposition to the tensile force carried by the cable.
The cable tethers the AMP to an anchor typically resting on the floor of the body of water, to moor the AMP. The cable is spooled on a winch which is typically onboard the AMP, and the depth of the AMP in the body of water is controlled by paying out, or reeling in, the cable by operation of the winch. Typically, while the AMP is cycled both upwardly and downwardly, measurements are made only during one-half of the cycle, i.e., either on the upward or downward stroke. It is important during the time that the measuring instruments are actively measuring the water that the AMP always travel in the same direction, preferably at a substantially constant speed.
In a calm or quiescent body of water, such control of the movement of the AMP is adequately provided simply by controlling the rate of turning of the spool, which has been the standard methodology. However, in many bodies of water, particularly the ocean, there are significant time-varying hydraulic pressure fields due to, e.g., turbulence, currents and wave orbitals. These pressure fields result in time and location dependent forces that act on the AMP in addition to the buoyant force and the force exerted by the cable. These forces can cause the AMP to stop its travel, or even to momentarily reverse itself and travel in the opposite direction. This can happen either on the upward or downward stroke. This results in un-even sampling, and where travel is stopped or reversed, can result in double-sampling, making some data unreliable or redundant and making interpretation of all the data more difficult.
The prior art has had two responses to this problem. One response has been to increase the buoyant force. If the buoyant force is large, then the forces produced by the pressure fields must also be large to cause the AMP to slow or change course.
A significant problem with this approach, however, is that to cycle the AMP, a power source is required to pull the AMP to maximum depth on its downward stroke. The larger the buoyant force, the more power is required. Where the power source is on-board the AMP as is preferred, increasing the power requirements either decreases the time that the AMP can provide for unattended service, or increases weight, which makes the AMP more difficult to deploy.
The other prior art response has been simply to use non-autonomous methods for profiling bodies of water with significant pressure fields, such as the ocean. For example, for profiling the ocean, the measuring instruments have typically been floated to the desired location in a boat, and manually lowered and then raised to obtain the measurements by personnel or equipment in the boat.
Accordingly, there is a need for a method and apparatus providing for improved control of the motion of an autonomous moored profiler.